Polytetrafluoroethylene polymers (PTFE) are well known for their superior heat resistance, chemical resistance, corrosion resistance and anti-stick properties. Because of these properties, PTFE has found a wide range of applications including the use in so-called non-stick coatings on metal substrates such as cookware including cook, bake and frying articles.
However, because of its excellent anti-stick properties, special measures need to be taken to provide good adherence of a PTFE coating to a metal substrate. Accordingly, the art has developed special coating systems to provide PTFE anti-stick coatings on metal substrates. Examples of such coating systems can be found in for example EP 894541, WO 02/14065, U.S. Pat. Nos. 5,160,791; 5,230,961; 5,223,343; 5,168,107 and U.S. Pat. No. 5,168,013. Anti-stick coatings on substrates such as metal and glass typically comprise of two or more layers although a single layer has been contemplated as well.
Typically, these coating systems include two layers consisting of a specially formulated primer and topcoat, but systems incorporating one or more intermediate coats are also known. The primers for such systems typically contain a heat resistant organic binder resin and one or more fluoropolymer resins, along with various opaque pigments and fillers. The intermediate coats contain mainly fluoropolymers with some amounts of opaque pigments, fillers and coalescing aids, while the topcoats are almost entirely composed of fluoropolymers.
EP 124085 discloses a primer coating composition that comprises: (a) a modified tetrafluoroethylene polymer in the form of particles of double-layer structure having a core and a shell, the core comprising a tetrafluoroethylene homopolymer or a copolymer of tetrafluoroethylene with a fluorine-containing α-olefin represented by the formulaCF2═CFRwherein R is a member selected from the group consisting of chlorine, and perfluoroalkyl, polyfluoroalkyl, perfluoroalkyloxy and polyfluoroalkyloxy each having 1 to 10 carbon atoms, and said alkyl or alkyloxy groups may have one or more oxygen atom in their chain, the shell comprising a copolymer of tetrafluoroethylene and the fluorine-containing α-olefin copolymerized therewith in a larger amount than in the core,    (b) a copolymer of tetrafluoroethylene and the fluorine-containing α-olefin uniformly copolymerized therewith, and    (c) an auxiliary adhesive agent selected from the group consisting of polyimide, polybismaleimide, polyamideimide and aromatic polyamide.
WO 02/14065 provides multiple coat non-stick coating systems, which include a primer, topcoat, and optionally, a midcoat, as well as substrates coated with the non-stick coating systems. The primer of the multiple coat system includes a fluoropolymer, which is a polymer including a CF2—CH2 moiety in the polymer chain. As an example thereof, a fluoropolymer copolymer is disclosed, such as a fluoropolymer terpolymer including repeating monomers of tetrafluoroethylene (TFE), hexafluoropropylene (“HFP”), and vinylidene fluoride (VDF).